MELBOURNE, Australia (AP):

Novak Djokovic held off a valiant Swiss player for a five-hour, five-set victory yesterday, extending his winning streak to 18 matches at the Australian Open and then ripping off his shirt to celebrate.

The big surprise: It was a fourth-round match against Stanislas Wawrinka of Switzerland, not a final against Roger Federer.

Djokovic edged the 15th-seeded Wawrinka 1-6, 7-5, 6-4, 6-7 (5), 12-10 in a momentum-swinging marathon, cashing in on his third match point to reach the quarterfinals for a 15th consecutive major tournament.

The style was reminiscent of his five-hour, 53-minute final win here last year against Rafael Nadal.

"He deserved equally to be a winner of this match," said Djokovic, who is aiming to be the first man in the Open era to win three consecutive Australian titles. "I give him a lot of credit. He has all my respect. He was the aggressive player on the court. I was just hanging in there trying to fight."

Djokovic next faces Tomas Berdych, the 2010 Wimbledon finalist who advanced with a 6-3, 6-2, 7-6 (13) win over South Africa's Kevin Anderson.

Fourth-seeded David Ferrer won 6-2, 6-1, 6-4 over No. 16 Kei Nishikori of Japan to set up an all-Spanish quarter-final against Nicolas Almagro, who was leading 6-2, 5-1 when No. 8 Janko Tipsaveric retired from their fourth-round match.

no trouble for Sharapova

Maria Sharapova has had almost no trouble on the women's side, beating Kirsten Flipkens of Belgium 6-1, 6-0 earlier yesterday to continue a dominant and unparalleled run in Melbourne.

The second-ranked Sharapova lost the 2012 women's final to Victoria Azarenka, but she has lost only five games in four matches on the way to the quarter-finals this year, an Australian Open record that seems immaterial to the 25-year-old Russian.

"Well, I'm certainly happy to be playing this well but ... it only gets tougher from here," said Sharapova, who is playing her first tournament of 2013 after withdrawing from a warm-up event in Brisbane because of an injured right collarbone.

Sharapova next plays fellow Russian Ekaterina Makarova, who ousted fifth-seeded Angelique Kerber 7-5, 6-4. Sharapova beat Makarova in the quarter-finals here last year.

Li Na, who reached the Australian Open final in 2011 and won the French Open later that year, saved a set point in the tiebreaker before beating Julia Goerges 7-6 (6), 6-1. She'll next play No. 4 Agnieszka Radwanska, who beat No. 13 Ana Ivanovic 6-2, 6-4 for her 13th consecutive win. Radwanska won the Auckland and Sydney titles before coming to Melbourne.